Week 7-8 Flashcards
Degradable Implant
does not have to be
removed surgically once it is no longer needed
Degradable polymers
value in short-term
applications that require only the temporary presence
of a device
The tissue engineering and regenerative medicine
usually require:
e biodegradable or bioeliminable
polymers in their preparation
Degradation
breakage of bonds
biodegradation, bioerosion, bioabsorption, and
bioresorption
being used to indicate that a given material or device will eventually
disappear after having been introduced into a living organism.
Erosion
physical changes in size, shape, or mass of a material, which could be the consequence of either degradation or simply dissolution.
Erosion can occur in the absence of degradation, and degradation can occur in the absence
of erosion.
A sugar cube placed in water erodes, but the sugar does not chemically
degrade.
* Likewise, the embrittlement of plastic when exposed to UV light is due to
the degradation of the chemical structure of the polymer and takes place
before any physical erosion occurs.
“bio” indicates that the erosion occurs under…..
physiological conditions(high temperature, strong
acids or bases, UV light, or weather conditions)
“bioresorption” and “bioabsorption”
polymer or its degradation products are removed by cellular activity in a biological environment.
“biodegradation”(Consensus Conference of the European Society for Biomaterials (Williams, 1987))
only when we wish to emphasize that a biological agent (enzyme,
cell, or microorganism) is causing the chemical degradation of the implanted device.
Why the degradation of poly(lactic acid) to lactic acid should not be described as biodegradation?
It is a simple hydrolysis and independently occurs of any biological agent.
the important properties
required of degradable polymers:
- It shouldn’t cause any acute or chronic inflammatory response
- The degradation time and change in the properties of the polymer
should match the healing of the tissue. - should not be toxic; they should be
metabolized and cleared from the body.
Degradation can also occur via
enzymatic mechanisms
Which bonds can provide biomaterials to be degradable?
Ester, amide etc. should be present. These bonds are cleavable.
Hydrolytic degradation is possible if:
water is abundant in the tissues
The degradation rate of hydrolytically cleavable bonds can be increased
substantially by:
enzymes
Two mechanisms exist for the hydrolytic degradation of polymers:
(i) bulk degradation that happens throughout the structure of the polymeric product,
and
(ii) surface erosion, where water cannot penetrate the bulk and its action stays
limited to the surface.
Bulk degradation occurs when:
the rate of water penetration is faster than
the rate of degradation. Initially the water penetrates the device and begins degrading
the entire device.
……………….. are the best known examples of
polymers that can be fabricated into surface eroding devices.
polyanhydrides and poly(ortho esters)
Structural parameters influencing the degradation rate of polymers
-Chemical composition
-Molecular weight and its distribution
-Presence of polar and ionic groups
-Presence of hydrophilic functional groups
-Presence of branches or side chains
Physical parameters influencing the degradation rate of polymers
-Shape and surface roughness of the device
-Inhomogenecity of the material
-Presence of micro or macro cracks on the device
-Processing and thermomechanical history
Environmental parameters influencing the degradation rate of polymers
-Sterilization process
-Implantation site
-Adsorbed or absorbed chemicals
Ionic strength and pH of the media
-Hydrolytic environment
-Enzymatic environment
Surface area and rate of degradation:
The higher the surface area, the higher is the rate of degradation.
The degradation of semicrystalline polymers proceeds in two phases;
; in the first phase,
the amorphous regions and then, in the second phase, the crystalline regions are
hydrolyzed
